This invention relates to vehicle detectors used to detect the presence or absence of a motor vehicle in an inductive loop mb dded in a roadbed. More particularly, this invention relates to a vehicle detector with an automatic loop checking capability.
Vehicle detectors have been used for a substantial period of time to generate information specifying the presence or absence of a vehicle at a particular location. Such detectors have been used at intersections, for example, to supply information used to control the operation of the traffic signal heads; have been used in railway installations for railway car detection and control; and have also been used to supply control information used in conjunction with automatic entrance and exit gates in parking lots, garages and buildings.
A widely used type of vehicle detector employs the principle of period shift measurement in order to determine the presence or absence of a vehicle in or adjacent the inductive loop mounted on or in a roadbed. In such systems, a first oscillator, which typically operates in the range from about 10 to about 120 kHz is used to produce a periodic signal in a vehicle detector loop. A second oscillator operating at a much higher frequency is commonly used to generate a sample count signal over a selectable, fixed number of loop cycles. The relatively high frequency count signal is typically used to increment a counter, which stores a number corresponding to the sample count at the end of the fixed number of loop cycles. This sample count is compared with a reference count stored in another counter and representative of a previous count in order to determine whether a vehicle has entered or departed the region of the loop in the time period between the previous sample count and the present sample count.
The initial reference value is obtained from one or more initial sample counts and stored in a reference counter. Thereafter, successive sample counts are obtained on a periodic basis, and compared with the reference count. If the two values are essentially equal, the condition of the loop remains unchanged, i.e., a vehicle has not entered or departed the loop. However, if the two numbers differ by at least a threshold amount in a first direction (termed the Call direction), the condition of the loop has changed and may signify that a vehicle has entered the loop. More specifically, in a system in which the sample count has decreased and the sample count has a numerical value less than the reference count by at least a threshold magnitude, this change signifies that the period of the loop signal has decreased (since fewer counts were accumulated during the fixed number of loop cycles), which in turn indicates that the frequency of the loop signal has increased, usually due to the presence of a vehicle in or near the loop. When these conditions exist, the vehicle detector generates a signal termed a Call Signal indicating the presence of a vehicle in the loop.
Correspondingly, if the difference between a sample count and the reference count is greater than a second threshold amount, this condition indicates that a vehicle which was formerly located in or near the loop has left the vicinity. When this condition occurs, a previously generated Call Signal is dropped.
The Call signals generated by a vehicle detector are used in a number of ways. Firstly, the Call signals are presented to an output terminal of the vehicle detector and forwarded to various types of traffic signal supervisory equipment for use in a variety of ways, depending on the system application. In addition, the Call signals are used locally to drive a visual indicator, typically a discrete light emitting diode (LED) or a multiple LED display or a liquid crystal display (LCD) to indicate the Call status of the vehicle detector, i.e. whether or not the vehicle detector is currently generating a Call signal.
Vehicle detectors with the Call signal generating capability described above are used in a wide variety of applications, including vehicle counting along a roadway or through a parking entrance or exit, vehicle speed between preselected points along a roadway, vehicle presence at an intersection controlled by a traffic control light system, or in a parking stall, in railroad yards, and numerous other applications.
In all applications, proper operation of the basic vehicle detector is entirely dependent upon the integrity of the loop to which the vehicle detector is connected and which is an integral part of the loop oscillator circuit. If the loop is electrically open or shorted the loop oscillator circuit cannot function in a predictable manner, and the sample counts obtained over the measurement periods will be totally inaccurate. As a direct consequence, the vehicle detector can not function in its required reliable manner. Although a vehicle detector loop is carefully checked immediately after installation in situ to ensure loop integrity, loop failure may occur anytime thereafter due to the severe environmental conditions within which such loops are expected to operate. More specifically, such loops are regularly subject to extreme temperature fluctuations (both short term and long term), and mechanical abrasions, and may be the object of random acts of vandalism or other destructive forces. Consequently, prudent maintenance of a vehicle detector system usually includes some provision for checking the integrity of the vehicle detector loop. In the past, such loop checks have required that a skilled maintenance technician visit the loop site and manually perform appropriate mechanical and electrical tests. When a defective loop is revealed by this procedure, the technician will either repair the mechanical or electrical problem or disable the vehicle detector until suitable repairs can be made. Before the advent of the instant invention these maintenance procedures have been considered to be necessary and unavoidable, even though such manual inspection and test routines are costly and time consuming, and thus relatively undesirable. More importantly, however, manual inspection and test maintenance is a relatively inefficient approach to the problem of loop maintenance due to the random nature of loop failures: in any given loop installation, the loop can fail mechanically or electrically moments after a service technician has checked and verified the mechanical and electrical integrity of a vehicle detector loop. Efforts in the past to provide vehicle detector systems devoid of the above disadvantages have not met with success.